Catalytic VOCs elimination over copper and cerium oxide modified mesoporous SBA-15 silica

Article abstract of DOI:10.1016/j.apcata.2012.12.007

Copper and cerium oxide bi-component materials with different Cu/Ce ratio were prepared using ordered SBA-15 silica as a support and compared with their bulk analogs. The samples were characterized by nitrogen physisorption, XRD, UV-Vis, FTIR, XPS, Raman spectroscopy and TPR with hydrogen. Cyclohexanol conversion was used as a catalytic test to obtain more information for the surface properties of the supported materials. The catalytic properties of the samples were studied in VOCs oxidation using toluene and ethyl acetate as probe molecules. A strong effect of mesoporous silica support and samples composition on the formation of catalytic sites was established.

Full text of DOI:10.1016/j.apcata.2012.12.007

Accepted Manuscript
Title: Catalytic VOCs elimination over copper and cerium
oxide modified mesoporous SBA-15 silica
Authors: Tanya Tsoncheva, Gloria Issa, Teresa Blasco,
Momtchil Dimitrov, Margarita Popova, Selene Hern a´ ndez,
Daniela Kovacheva, Genoveva Atanasova, Jos e´ M. L o´ pez
Nieto
PII:
DOI:
Reference:
S0926-860X(12)00756-9
doi:10.1016/j.apcata.2012.12.007
APCATA 13998
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
21-9-2012
26-11-2012
5-12-2012
Please cite this article as: T. Tsoncheva, G. Issa, T. Blasco, M. Dimitrov, M. Popova,
S. Hern a´ ndez, D. Kovacheva, G. Atanasova, J.M.L. Nieto, Catalytic VOCs elimination
over copper and cerium oxide modified mesoporous SBA-15 silica, Applied Catalysis
A, General (2010), doi:10.1016/j.apcata.2012.12.007
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*Graphical Abstract (for review)
Ethyl acetate oxidation
100
bulk
80
60
40
20
0
SBA-15
T=630 K
0
33
66
99
Ce/Cu+Ce, wt.%
Page 1 of 42

*Highlights (for review)



The mode of copper-ceria interaction regulates catalytic activity
Porous silica support controls the interaction between metal oxide particles
Silica porous support changes the effect of Cu/Ce composition.
Page 2 of 42

*Manuscript
Catalytic VOCs elimination over copper and cerium oxide modified
mesoporous SBA-15 silica
a
a
b
a
a
Tanya Tsoncheva , Gloria Issa , Teresa Blasco , Momtchil Dimitrov , Margarita Popova ,
b
c
c
b
Selene Hernández , Daniela Kovacheva , Genoveva Atanasova , José M. López Nieto
a
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of
Sciences, Sofia, Bulgaria, e-mail:tsoncheva@orgchm.bas.bg, tel.+029796640
b
Instituto de Tecnologia Química (UPV-CSIC), Campus Universidad Politécnica Valencia,
6022-Valencia, Spain
4
c
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G.
Bonchev St., Block 13, 1113 Sofia, Bulgaria.
Abstract
Copper and cerium oxide bi-component materials with different Cu/Ce ratio were prepared
using ordered SBA-15 silica as a support and compared with their bulk analogues. The
samples were characterized by nitrogen physisorption, XRD, UV-Vis, FTIR, XPS, Raman
spectroscopy and TPR with hydrogen. Cyclohexanol conversion was used as a catalytic test to
obtain more information for the surface properties of the supported materials. The catalytic
properties of the samples were studied in VOCs oxidation using toluene and ethyl acetate as
probe molecules. A strong effect of mesoporous silica support and samples composition on
the formation of catalytic sites was established.
Key words
VOCs oxidation, copper and cerium oxide catalysts, SBA-15 support, catalytic sites
1
Page 3 of 42

1
. Introduction
Volatile organic compounds (VOCs) emitted from various industrial processes and transport
activities are considered as an important class of air pollutants, and now they are a subject of
strict legislation. In the last decades, many technologies have been devoted for the abatement
of VOCs emissions. Among them, the catalytic total oxidation has been recognized as more
economical process for VOCs elimination even in low concentrations, reducing also the level
of toxic by-products emission in a high extent [1-4]. Transition metal oxide catalysts offer
much technical and commercial advantage that make them a viable alternative to noble metals
[
5-11].
Recently, copper cerium oxide binary system have been intensively studied in various
reactions, such as combustion of CO and methane [1, 12-15], SO reduction [16], hydrogen
2
production by steam reforming of methanol [17, 18], reduction of NO [19], decomposition of
H O [20], wet oxidation of phenol [21] etc. It was demonstrated higher catalytic activity of
2
2
copper cerium mixed oxides than precious metals for preference CO oxidation in excess of
hydrogen (CO-PROX) [12, 22-25] as well as excellent activity in water gas shift reaction
(WGSR) at low temperature [26-29]. To date, these materials are also used as additives in the
three way catalysts for purification of automobile exhaust gas in order to reduce the cost of
noble metals [30 and refs. therein]. The superior catalytic activity of this binary system in
different reactions has been a subject of intensive investigation and controversial conclusions.
Generally, a synergism of the redox properties of both metal oxides is assumed and it is
assigned to strong interaction between CuO and CeO , which causes part of CuO to enter in
2
ceria lattice, forming solid solution, and other part to disperse over the surface [25,31-34].
Some authors related the catalytic behaviour of copper modified ceria with the creation of
additional surface oxygen vacancies [35, 36] and assumed that the dispersion and exposure of
2
Page 4 of 42

nanosized copper particles in close contact with ceria vacancies play an essential role for
WGSR at low temperature [28,37,38]. The other authors considered that finely dispersed CuO
are the main active sites in CO oxidation and the larger bulk CuO nanoparticles possess a
+
little contribution in the catalytic reaction [39]. Alternatively, the stabilization of Cu ions
due to the strong interaction between copper oxide clusters and ceria and their activity in CO
oxidation was reported [25]. Martinez-Arias et al. [23] claimed that the facile redox interplay
between cerium and copper is a key factor of high catalytic activity of these binary samples.
Boaro et al. [40] related the highest activity of the interface between both metal oxide phases
to intensive mobility of surface oxygen and changes in the redox state of both copper and
cerium phases. Recently Dziembaj et al. [41] assumed that the mobility of oxygen and the
electron transfer within the bulk of ceria must be also taken into consideration during the
oxidation reactions.
Although copper-cerium catalysts have been considered as effective oxidation catalysts, their
performance in VOCs oxidation has been scarcely studied. Delimaris [1] and Hu [3]
investigated the catalytic performance of copper ceria binary oxides in acetone, ethanol, ethyl
acetate and toluene combustion. They demonstrated an optimum in copper content to achieve
excellent catalytic activity in ethanol and ethyl acetate combustion, while no significant effect
of copper addition to ceria was observed at low temperatures for toluene oxidation. The
authors concluded that the active sites that are responsible for CO oxidation did not operate
under the conditions of VOCs oxidation due to the complexity of reaction pathway for the
latter materials and the increase of the catalytic activity was considered as a result of specific
surface area increase in binary system as compared to the corresponding single metal oxides.
Hu [42,43] related the higher activity of copper cerium catalysts in acetone oxidation to the
formation of highly dispersed copper oxide particles, but they did not exclude the positive
+
effect of Cu ions as well. Saqer et al. [44] assumed that the large CuO crystallites did not
3
Page 5 of 42

contribute significantly to the catalytic activity in toluene oxidation of alumina-supported
copper ceria binary oxides. They reported that the improved performance of mixed binary
oxide materials is not due to the formation of solid solution or new mixed oxide system, but
due to facile spreading of metal oxides over each other [44].
Despite the copper ceria catalysts have been intensively studied, most of investigations were
focused on their behaviour in bulk, non-supported form, where copper oxide was doped with
ceria or vice versa. It was found that the intimate contact between different metal oxide
particles depends on the amount of both components as well as on the preparation method and
the activation procedures used [30]. The behaviour of these materials, when they were loaded
on various supports was studied in much lower extent [4, 30, 45]. Rao [32] reported higher
interaction between different oxide species when they were supported on SiO and ZrO
2
2
support as compared to the Al O support, which provided higher activity in CO oxidation.
2
3
Ordered mesoporous silicas have been assumed as suitable catalysts support because
mesopores provide large specific surface area and porous volume for achievement high
metal/metal oxides loading. During last two decades, the success in the synthesis of
mesoporous silicas resulted in preparation of large family of mesoporous silicas with tunable
pore size, shape and topology, which opened new prospects for the fabrication of stable, well
dispersed, and uniform metal/metal oxide nanoparticles via their hosting into the mesoporous
matrix [46]. However, to the best of our knowledge, negligible data for the simultaneous
hosting of copper and cerium oxide species on mesoporous silica supports were published
[
45].
The aim of present paper is to study a series of binary copper and cerium oxide materials with
different composition, supported on mesoporous SBA-15 silica. The effect of porosity on the
state of loaded metal oxide nanoparticles was studied using a complex of various
physicochemical techniques, such as XRD, nitrogen physisorption, TPR with hydrogen,
4
Page 6 of 42

FTIR, UV-Vis and XPS spectroscopy as well as cyclohexanol conversion as a catalytic test.
In order to study the nature of the loaded species, the supported copper ceria materials were
compared with their bulk analogues. They were obtained by co-precipitation technique, since
it has been considered as the preferment method for synthesis of copper doped ceria with
good intimate contact between different particles [22, 30, 35, 47]. The obtained binary metal
oxide materials were studied as potential catalysts for total oxidation of different VOCs
pollutants. Ethyl acetate and toluene were chosen as model compounds due to their hazardous
nature and extensive use as solvents and feedstock in printing, pressing, petro chemistry and
other industries.
2
2
. Experimental
.1. Materials
SBA-15 mesoporous silica was prepared according to the procedure described in [48] using
Pluronic 123 triblock-co-polymer (EO PO EO ) as structure-directing agent and TEOS
2
0
70
20
(Aldrih) as a siliceous source. The hydrothermal synthesis was performed at 368 K and the
template was released by calcinations in air at 773 K for 6 hours. Copper and cerium
modifications were prepared by incipient wetness impregnation of mesoporous silica with
aqueous solutions of Cu(NO ) .2H O or Ce(NO ) .6H O for the mono-component materials
3
2
2
3 3
2
(
CuO/SBA-15 and CeO /SBA-15, respectively), or with a mixture of both precursors in
2
appropriate ratio, for the bi-component ones. The nitrate precursors were decomposed after
pre-treatment in air at 773 K for 2 h. The total metal content in all modifications was 6 wt%.
In the case of bi-component materials, the Cu/Ce weight ratio changed as 2/4, 3/3 and 4/2 for
the 2Cu4Ce/SBA-15, 3Cu3Ce/SBA-15 and 4Cu2Ce/SBA-15 samples, respectively. For
comparison, mono- and selected bi-component bulk analogues of these materials were
obtained by precipitation of an aqueous solution of the corresponding salts, i.e. Ce- and Cu-
5
Page 7 of 42

nitrates, with KOH (2 M), filtered, dried at 373K and calcined at 773 K for 2h. The samples
were denoted as CuO and CeO , for the mono-component materials and 2Cu4Ce and 4Cu2Ce,
2
for the bi-component ones with Cu/Ce weight ratio of 2/4 and 4/2, respectively.
2
.2. Methods of investigation
Nitrogen physisorption data were obtained by multi-point N adsorption at 77 K, in a
2
Micromeritics ASAP 2000 instrument. Powder X-ray diffraction patterns were collected
o
o
within the range of 5.3 to 80 2θ with a constant step 0.02 2θ on Bruker D8 Advance
diffractometer with Cu K radiation and LynxEye detector. Low-angle part of the patterns was
α
o
collected from 0.3 to 8 2θ using the knife-edge anti-scatter screen attachment of the primary
beam. Phase identification was performed with the Diffracplus EVA using ICDD-PDF2
Database. The average metal oxide crystallite size was evaluated from the diffraction peaks
0
0
broadening at 28.5 2θ for ceria and 35.4 2θ for copper oxide, respectively, according the
Scherrer equation. The IR spectra (KBr pellets) were recorded on a Bruker Vector 22 FTIR
−
1
spectrometer at a resolution of 1–2 cm , accumulating 64–128 scans. The UV–Vis spectra
were recorded on the powder samples using a Jasco V-650 UV-Vis spectrophotometer
equipped with a diffuse reflectance unit. The XPS measurements were done in the UHV
chamber of ESCALAB- Mk II (VG Scientific) electron spectrometer with a base pressure of
-8
1
.10 Pa. The photoelectron spectra were excited using unmonochromatized Al Kα1,2
radiation (h = 1486.6 eV). The composition and chemical surrounding of samples were
investigated on the basis of the areas and binding energies of Cu2p , Ce3d and Si2p
3
/2
photoelectron peaks and Scofield’s photoionization cross-sections. Raman spectra were
recorded in ambient conditions using a Renishaw system 1000 ‘‘in via’’ attached to a
microscope. An argon ion laser (532-nm) was used as the excitation source and was typically
operated at a power of 20 mW.
6
Page 8 of 42

The TPR/TG (temperature-programmed reduction/ thermogravimetric) analyses were
performed in a Setaram TG92 instrument. Typically, 40 mg of the sample were placed in a
3
-1
microbalance crucible and heated in a flow of 50 vol% H in Ar (100 cm min ) up to 773 K
2
-1
at 5 K min and a final hold-up of 1 h. The weight loss during the reduction was calculated on
the base of TPR-TG curves and the values were normalized to one and the same catalyst
weight (40 mg).
2
.3. Catalytic experiments
The catalytic oxidation of ethyl acetate was performed in a flow type reactor (0.030 g of
catalyst, 3 fold diluted with crash glass) with a mixture of ethyl acetate (1.2 %) in air (170
ml/min). In order the comparison of the samples to be more correct, the bulk materials were
used as a mechanical mixture with pure silica, following the composition of SBA-15
modifications. Before the catalytic experiments the samples were pre treated in argon at 373
K for 1 h. The catalytic experiments were performed with a stepwise temperature increase.
Data for the catalyst behaviour of the samples with time on stream at 630 K were also
collected. Gas chromatography analyses were done on a HP 5890 apparatus using carbon-
0
0
based calibration. Total conversion was calculated as X =(C EtAc-CEtAc)/C EtAc*100 and the
tot
0
0
yield of the i product as X =C /C EtAc*100, where C
CEtAc and C were the amount of ethyl
i
i
i
EtAc,
acetate in the initial flow, current amount of non-converted ethyl acetate and the amount of
the i product, respectively. The products distribution was calculated as CO (SCO2),
2
acetaldehyde (S ), ethanol (S ) and acetic acid (SAcA) selectivity by the equation: Si=
AA
Et
X /X *100, where Si and Xi are the selectivity and the yield of (i) product and X is the
i
tot
tot
conversion. The specific activity (SA) was calculated as the current conversion value per 1
specific surface area.
7
Page 9 of 42

Toluene oxidation was studied in a flow reactor using 0.030 g of catalyst, air as carrier gas (30
ml/min) and toluene partial pressure of 0.9 kPa. On-line gas chromatographic analysis were
performed on HP 5980 with PLOT Q column using detector of thermo conductivity. The
catalytic activity was studied in temperature programmed regime. Total conversion was
0
0
0
tol
calculated as X =(C -C )/C *100 and the yield of the i product as X =C /C *100,
tot
tol tol
tol
i
i
0
where C C and C were the amount of toluene in the initial flow, current amount of non-
tol, tol
i
converted toluene and the amount of the i product, respectively. The selectivity to CO (SCO2
)
2
was calculated as Si= XCO2/X *100. The samples were tested also with time on stream under
tot
isothermal conditions (630 K).
Cyclohexanol conversion was carried out in a flow reactor (0.030 g of catalyst), using
nitrogen as carrier gas and nitrogen to cyclohexanol molar ratio of 38. The activity was
determined at 523 K after the samples pre-treatment in air at 373 K or reduced in hydrogen at
5
3
23 K for 2 h. On-line analysis of the reaction products was performed using HP-GC with a
0 m HP-35 capillary column. The results were presented as selectivity to cyclohexene and
cyclohexanone, calculated as a ratio between the current yields of the corresponding products
and total conversion (%) at the selected temperature.
3
3
3
. Results and Discussion
.1. Catalytic properties in VOCs oxidation
.1.1. Ethyl acetate oxidation
The variation of catalytic properties in the oxidation of ethyl acetate with reaction temperature
of copper and cerium oxide non-supported (bulk) and supported on SBA-15 materials are
presented in Fig 1. All catalysts exhibit catalytic activity above 570 K (Figs. 1a and 1c).
CuO/SBA-15 demonstrates relatively low conversion degree (below 40 %) in the whole
investigated temperature interval, while the non-supported CuO reveals much higher catalytic
8
Page 10 of 42

activity with the temperature increase above 600 K. Both ceria materials show significantly
higher catalytic activity even at relatively low temperatures and again, it is better for the bulk
analogue. 4Cu2Ce sample exhibits higher catalytic activity and CO selectivity than the
2
corresponding mono-component bulk materials (Fig. 1). Just the opposite behavior is
observed for the bulk bi-component sample with lower copper content (2Cu4Ce). Such a
strong dependence of the samples composition is not observed for the supported bi-
component analogues (Fig. 1c). Thus, the best activity is found for 2Cu4Ce/SBA-15, while
the other bi-component samples are more active only than CuO/SBA-15. On the other hand,
carbon dioxide was the main reaction products, especially at high reaction temperatures
(Figs.1b and 1d).
Generally, the catalysts demonstrate a slight tendency of conversion decrease with time on
stream at 630 K, the effect being most pronounced for the samples with higher copper content
(
Fig 2a and 2d). In all case, the selectivity to CO strongly increases when increasing ethyl
2
acetate conversion (Fig 2b and 2e). Moreover, other reaction products such as ethanol,
acetaldehyde and acetic acid with different selectivity are registered during the oxidation of
ethyl acetate (Figs. 2c and 2f).
3
.1.2 Toluene oxidation
Catalytic properties in deep oxidation of toluene of SBA-15 supported Cu-Ce oxide catalysts,
chosen as a probe molecule of aromatic VOCs, are presented in Fig. 3. CO is observed as the
2
main reaction product in the whole investigated temperature interval (Fig. 3b) although, in
some cases, benzaldehyde is also registered as a by-product. All bi-component samples
exhibit higher catalytic activity in comparison with the mono-component ones but, at lower
temperature, higher ability for by-products formation is found. Note that CuO/SBA-15
9
Page 11 of 42

exhibits extremely fast deactivation with time on stream, while the activity of bi-component
samples remains relatively stable at 630 K (Fig. 3c and 3d).
3
3
.2. Catalysts characterization
.2.1. Nitrogen physisorption
In order to obtain information for the textural characteristics of the studied materials, nitrogen
physisorption measurements are done. The isotherms of parent SBA-15 and its metal oxide
modifications are of IV type with H1 hysteresis loop and sharp capillary condensation step in
the range of 0.5-0.8 P/P (supplementary data, Fig. S1), typical of mesoporous materials with
0
uniform cylindrical pores and average pore diameter about 5.4-5.8 nm [48]. The similarity of
the isotherms after the metal oxides loading reveals the preservation of support pore structure
during the modification procedure. The observed decrease in the BET surface area and pore
volume for the modified materials in comparison with the parent mesoporous support (Table
1
) could be due to location of metal oxide nanoparticles within the porous structure. The
change in the shape of desorption branch, combined with significant decrease in the
mesoporous volume (supplementary data, Fig. S1) could be due to location of metal oxide
phase into the mesopores [49]. This effect is more pronounced for 2Cu4Ce/SBA-15, where a
simultaneous decrease in the pore diameter, microporous volume and BET surface area is
observed. The observed features for the last sample does not exclude also partial location of
metal oxide particles within the support micropores.
In the case of bulk materials, the specific surface area (Table 1) for CeO is about 20 times
2
higher than for CuO. A decrease of BET surface area is observed after ceria modification with
copper. This effect is more pronounced for the sample with lower copper content (2Cu4Ce),
which suggests that the obtained bi-component bulk materials are not a physical mixture of
CuO and CeO oxides.
2
1
0
Page 12 of 42

3
.2.2. Powder X-ray diffraction
Small-angle X-ray diffraction patterns of parent SBA-15 silica and its modifications present
intensive well-resolved signals (supplementary data, Fig. S2), which can be indexed as (100),
(110) and (200) reflections of well organized 2-D hexagonal lattice with p6mm symmetry
[
48]. These results are in accordance with the nitrogen physisorption data indicating
preservation of support structure during the modification procedure. After the impregnation,
the intensities of the reflections decrease, which could be assigned to the decrease in electron
density contrast upon introduction of metal oxide species into the mesopores of the silica host
matrix.
0
In the wide angle region of CeO /SBA-15 pattern (Fig 4a), the broad reflections at 2θ= 28.5 ,
2
0
0
0
0
3
3.1 , 47.5 , 56.3 and 69.4 demonstrate the presence of small ceria crystallites (Table 1),
which in accordance with the nitrogen physisorption data (See Section 3.2.1), are
predominantly located within the mesopores of the support. The sharp diffraction peaks in the
0
0
0
0
0
0
0
CuO/SBA-15 sample at 2θ= 35.4 , 38.9 , 48.6 , 58.3 , 61.4 , 66.1 and 68.1 are due to the
presence of well crystallized CuO phase, probably located on the outer surface of mesoporous
support because of the relatively large crystal size (Table 1). The reflections of ceria are not
well resolved in all bi-component materials, and no reflections of copper oxide phase are
registered for 2Cu4Ce/SBA-15, evidencing the presence of finely dispersed metal oxide
particles, probably located into the porous structure (Table 1). At the same time, partial
segregation of CuO phase on the outer surface of the silica support is assumed for the bi-
component materials with higher copper content, i.e 4Cu2Ce/SBA-15 and 3Cu3Ce/SBA-15
(Table 1, Fig. 4a).
More finely dispersed CuO and CeO metal oxide particles in comparison with the
2
corresponding pure metal oxides is observed for the bulk bi-component materials (Fig.4b,
1
1
Page 13 of 42

Table 1). Here, a slight increase of the ceria unit cell parameter is also registered (Table 1).
However, the observed results are insufficient to assume or rule out the formation of a solid
4
+
2+
solution since according to [38], the substitution of large Ce ion (0.97 nm) by smaller Cu
ion (0.73 nm) is expected to contract the ceria cell, while the concomitant formation of
oxygen vacancies provide the opposite effect and the combination of both factors results in
preservation of unit cell parameters to a high extent.
The XRD patterns of the catalysts tested in the ethyl acetate oxidation suggest the formation
of reduced copper species like Cu O and metallic copper in copper-rich catalysts (Fig. 4,
2
Table 1).
3
.2.3. FTIR and UV-Vis spectra
In order to obtain more information for the state of various metal oxide species, FTIR and
UV-Vis spectra are recorded. FTIR spectra of all parent and modified SBA-15 silica materials
-1
consist of intense bands at around 1080, 800 and 450 cm (supplementary data, Fig. S3),
which are typical of the characteristic symmetric and asymmetric vibrations in Si-O-Si
-1
bridges in silicas [50]. The band around 1600 cm is due to the adsorbed water molecules
-1
[
51]. The broad band in the interval 3100-3700 cm is assigned to O-H stretching vibrations.
-1
The additional bands in the 600-400 cm region, which are well resolved in the spectra of
CuO/SBA-15, can be assigned to the presence of CuO crystallites [52]. However, these bands
are in a high extent masked by the intensive bands of the silica matrix and we can not observe
-1
them for samples with lower copper content. The interpretation of the band around 960 cm is
more complicated and it is assigned generally to Si-O stretching vibrations in defect Si-O-M
structures, where M is hydrogen or metal ion [53]. Despite the controversial interpretation of
this band [53-56], the significant changes observed in its profile only for CuO/SBA-15 could
be tentatively assigned to the existence of strong interaction between copper species and
1
2
Page 14 of 42

support silanol groups (supplementary data, Fig. S3). This interaction seems to decrease for
bi-component materials, probably due to the creation of new contact between both metal
oxide nanoparticles.
The diffuse reflectance UV- Vis spectra are further recorded, since this technique is very
sensitive to obtain more information for the environment of metal ions (Fig. 5). The bands at
3
43 and 313 nm, which are registered in the spectra of CeO and CeO /SBA-15 samples,
2 2
2
-
4+
respectively, correspond to O →Ce CT transitions [57]. The observed shift of the bands for
the latter material is due to the formation of smaller ceria particles after their deposition on
SBA-15 [58,59], in accordance with the XRD data (See Section 3.2.2). The band at 250 nm
2
-
3+
could be due to the O →Ce CT transitions, which implies the occurrence of oxygen
vacancy defects [60,61]. The absorption in the range of 240-320 nm and 600-800 nm regions
2
-
2+
for CuO and CuO/SBA-15 is related to O →Cu CT and d-d transitions, respectively, of
crystalline CuO [57]. The position of the peak at around 240 nm is slightly blue shifted for
CuO/SBA-15, which is probably due to the presence of more finely dispersed CuO crystallites
[
62]. The spectra of all bi-component materials represent higher absorption in the 300-550 nm
region (Fig. 5). This could be assigned to higher particles dispersion and the appearance of
strong interaction between them. According to [24], the absorption increase in this region
+
does not exclude the formation of Cu ions. The interpretation of the band at about 250 nm,
3+
which is generally assigned to Ce ions, is also quite complicated due to its overlapping with
the typical bands of CuO in this range.
3
.2.4. Raman spectroscopy
In order to obtain more information for the type of interaction between metal oxide
nanoparticles, Raman spectra are obtained (Fig. 6). The spectrum of the CuO sample (Fig. 6a)
-1
presents three main peaks at 275, 330 and 619 cm , typical of CuO phase [63]. The spectrum
1
3
Page 15 of 42

-
1
of CeO consists of intense peak at ca. 463 cm , which is assigned to F2g mode of CeO
2
2
fluorite structure [18, 43, 64, 65]. A slight red shift of the position of this band, combined
with significant decrease in its intensity with a simultaneous increase of the half-width is
observed for 2Cu4Ce (Fig. 6a). At the same time the disappearance of the main CuO peaks
-1
and a significant increase of the relative intensity of the peak at ca. 600 cm is observed.
Generally, the latter feature is assigned to the formation of oxygen vacancies due to the
4
+
replacement of Ce by metal ion with different valence [36, 63, 66, 67].
The Raman spectrum of pure SBA-15 silica support represents the main peaks at around 500,
-1
6
04, 810 and 960 cm , which could be assigned to the vibrations of three and four silane
rings, siloxane linkage and defect sites, such as silanol groups (Fig. 6b) [68]. The main Raman
-
peak of ceria in CeO /SBA-15 (Fig. 6b) becomes broader, more asymmetric and about 10 cm
2
1
red shifted in comparison with bulk CeO , probably due to the formation of more finely
2
dispersed ceria nanoparticles within the porous silica structure [66]. The main peaks of copper
oxide are well resolved in the spectrum of CuO/SBA-15, which supports XRD data for the
formation of well crystallized CuO nanoparticles. Note the increased intensity of the band
-1
around 960 cm for CuO/SBA-15 in comparison with the parent mesoporous silica support.
In accordance with the FTIR and UV -Vis data, we assign this feature to the presence of
finely dispersed copper oxide species, which are in strong interaction with surface silanol
groups. This high intensity band is still present in the spectrum of 2Cu4Ce/SBA-15, while the
bands which are typical of CuO phase fully disappear. In accordance with the UV-Vis data
(Fig.5b), we assume the influence of ceria to the formation of finely dispersed copper
containing species, which are in strong interaction with the support.
3
.2.5. XPS spectra
1
4
Page 16 of 42

More information for the concentration and oxidative state of different metal ions on the
catalyst surface is obtained by XPS measurements. The core level Cu2p3/2 spectra (Fig. 7a)
demonstrate shake-up peaks at 940-945 eV and main peak centered at ca. 932.8 eV. The main
Cu2p3/2 peak for 2Cu4Ce/SBA-15 is shifted to higher BE (933.2 eV), which is due to higher
dispersion of CuO in this sample [63,69]. The asymmetry of the main Cu2p3/2 peak to higher
BE, could be also due to the formation of Cu-O-Si surface structures [69]. Generally, the
presence of shake-up peak and the higher values of Cu2p3/2 BE (933.0-933.8 eV) is reported
2
+
as two major characteristics of Cu ions, while lower Cu2p BE (932.2-933.0 eV) and the
3
/2
absence of shake-up peak are characteristic for reduced copper species [6,70,71]. It is difficult
+
0
to distinguish Cu and Cu on the base of Cu 2p peaks alone, because they have essentially
3
/2
the same BE. That’s why the copper LMM Auger lines are also recorded (Fig. 7b). The broad
features, which are shifted to lower kinetic energy for the bi-component materials, could be
+
assigned to the presence of Cu ions, as was observed by number of authors [43,71-74]. Maluf
+
et al. [66] proposed that the formation of Cu might be induced at the interface by the
4
+
+
substitution of the two oxide phases, because Ce and Cu have similar radii [3 and refs.
therein]. Ce3d core level spectra are shown in Fig. 7c. The core level signals are broad due to
4
+
the contribution of several peaks, which could be assigned to Ce ions [33,72,73]. The bands
3
+
at ca. 903.2 and 884-886 eV, which are related to Ce ions [70], are also observed.
The surface concentrations of copper and cerium for the mono-component materials are lower
than the expected loaded ones (Table 2), which can be due to the formation of large metal
oxide particles or to the location of particles deeply into the mesopores of the support. Cu/Ce
ratio for 2Cu4Ce/SBA-15 increases above the loaded one and it could be related to the
increase in copper oxide dispersion, when it is in close contact with ceria species [43]. Just the
opposite effect is observed for 4Cu2Ce/SBA-15, which in accordance with the XRD data, is
due to the segregation of CuO phase with low degree of dispersion.
1
5
Page 17 of 42

After the catalytic test, reduction transformations of copper species are clearly observed for
all copper containing samples (Fig. 7, Table 2). Moreover, for bi-component materials, these
transformations are combined with a decrease in the Cu/Ce surface ratio and a simultaneous
4
+
increase in the relative part of Ce ions.
3
.2.6. TPR-TG measurements
In order to study the nature of metal oxide species in detail, TPR measurements are carried
out. This method is known as a powerful approach to obtain more information for the
environment of metal ions and it could be a key to understand the catalytic behaviour of
materials in various redox processes. Many studies demonstrate that the synergistic effect
between CuO and CeO is evident from the decrease in the reduction temperature of both
2
CuO and CeO oxides. However, we should note that the interpretation of the results from the
2
TPR analyses could be rather complicated, especially for the supported binary materials,
where interaction of different metal oxide species with the support and in-between is
expected. That’s why the interpretation of the reduction profiles of copper-cerium oxides
reported in the literature is rather controversial. Rao [74] observes the appearance of two
reduction peaks, centered at 418 and 443 K, which are assigned to the reduction of CuO
particles with different dispersion on ceria. Alternatively, these features are assigned to the
step-wise reduction of CuO [43,75]. Two reduction maxima are observed also by Liu [18], the
first one, whose intensity and position is not significantly varied with copper loading, is
attributed to copper ions or CuO clusters in close interaction with ceria. Moreover, the main
peak, whose intensity increases with copper loading, is attributed to the reduction of bulk-like
CuO. These authors reveal that the position of the first peak is representative for the strength
of copper cerium interaction. Hu [43] and Si [38] assume that highly dispersed copper clusters
or isolated ions, which are in strong interaction with the support, could be reduced at
1
6
Page 18 of 42

relatively high temperatures. Zheng [63] registered three reduction steps, assigned to the
reduction of highly dispersed copper species, strongly interacted with ceria and to the larger
CuO particles, less associated with ceria, respectively. Ramaswamy [24] also observed three
reduction peaks with maxima at around 420, 500 and 700 K, which are ascribed to the
reduction of finely dispersed CuO species in close contact with ceria, bulk CuO or clusters
loosely bonded to ceria, and reduction of ceria, which is facilitated by the presence of copper.
Avgouropoulos [76] observed four reduction effects, which are assigned to the reduction of
non crystalline CuO species, strongly interacted with ceria, larger CuO particles weakly
interacted with ceria and bulk CuO particles, respectively.
TPR- TG and TPR-DTG profiles of all studied SBA-15 supported materials are presented in
Figs. 8a and 8b. In order to understand the TPR effects better, data for their bulk analogues
are also obtained (Figs. 8c and 8d). Some TPR parameters are also listed in Table 3. The bulk
CuO sample exhibits a strong weight loss effect (Figs. 8c and 8d) above 433 K with a
2
+
maximum at about 471 K, which corresponds to total one step reduction of Cu to metallic
copper (Table 3). For the supported CuO/SBA-15 (Figs. 8a and 8b), the position of the main
reduction peak is almost similar to that one of the bulk material, but the peak is asymmetric to
higher temperatures and additional weight loss above 660 K is observed. Taking into account
the lower reduction degree (about 85%) for this material in comparison with its bulk analogue
and in accordance with the XRD, FTIR, Raman and UV-Vis data, we assume formation of
copper species with different dispersion on silica support, part of them being in strong
interaction with the support silanol groups. According to [77-80], a step wise
2
+
+
0
+
Cu →Cu →Cu reduction with partial stabilization of Cu ions on the silica surface is not
excluded for the smaller species. The reduction of both mono-component ceria materials starts
4
+
3+
above 660 K. The total reduction degree of Ce →Ce transition is about 3 times higher for
1
7
Page 19 of 42

the supported material, despite in both cases, it remains significantly low, probably due to the
reduction transformations of cerium ions predominantly from the surface [83].
The TPR profiles of bi-component bulk materials (Fig. 8c and 8d) are broader in comparison
with the mono-component bulk CuO and they are shifted to lower temperatures for the sample
with higher Cu/Ce ratio. We should also stress on the significantly lower reduction degree for
binary bulk materials than the expected theoretical one (Table 3). We assign these features to
the reduction of different copper containing species in these samples. In accordance with
XRD, UV-Vis, XPS and Raman data, the copper particles, which are easily reducible in the
sample 4Cu2Ce, are most probably finely dispersed CuO crystallites in close contact with
ceria. The copper ions, which are hardly reducible, are probably penetrated into the bulk of
ceria lattice, which seems to be dominant in the sample with lower Cu/Ce ratio (2Cu4Ce).
All binary supported systems reduce at lower temperatures than CuO/SBA-15 and behave like
4
Cu2Ce bulk sample (Figs. 8a and 8b). So, the main TG effect in them, which generally
occurs above 370 K with a maximum in DTG curves at around 460 K, is probably due to the
reduction of well crystallized and finely dispersed CuO crystallites. Note, the continuous
weight loss in the higher temperature interval (500-700 K) in the TG profiles for all bi-
component SBA-15 supported samples, which is not observed for CuO/SBA-15 and also for
the bi-component bulk materials. In accordance with Raman, UV-Vis, FTIR and XPS
measurements, we assign these features to the reduction transformations of 2-dimensional Cu-
O-Cu structures, stabilized by interaction with ceria or silica matrix.
3
.2.7. Cyclohexanol conversion as a catalytic test.
More information for the surface properties of the metal oxide species supported on SBA-15
is obtained using cyclohexanol conversion as a catalytic test (Fig. 9). It is well known the
flexibility of this molecule to dehydrate to cyclohexene in presence of catalytic acidic sites or
1
8
Page 20 of 42

to dehydrogenate to cyclohexanone on the redox sites. In our experiment, the selectivity to
cyclohexene and cyclohexanone is compared at 503 K, where the conversion degree over all
materials is almost similar (about 20 %).
Cyclohexene is the major product for all supported materials, indicating the presence of acidic
sites (Fig. 9a), which must be higher for all bi-component materials, according with the higher
selectivity to cyclohexene. After reduction of the samples at 523 K (Fig. 9b), a significant
decrease of acidity is observed only for the samples with high copper content (CuO/SBA-15
and 4Cu2Ce/SBA-15), while the acidity of pure ceria (CeO /SBA-15) and 2Cu4Ce/SBA-15
2
materials remains almost unchanged. So, the activity of the catalysts depends predominantly
on the state of copper ions in them and then to the presence of Lewis acidic function, which
must be associated either to copper ions in low coordination in finely dispersed copper oxide
[
77, 80], or to the presence of oxygen vacancies generated by substitution of cerium ions by
copper.
Physicochemical characterization of the bi-component catalysts suggests that there must be
interaction between the metal oxide particles, and that this is favored for the supported
materials due to higher ceria dispersion. The interaction of copper oxide with ceria generates
copper species whose nature and concentration depends on the Cu/Ce ratio and also differ for
the bulk and supported samples with the same metal content. The presence of copper species
different to CuO in bi-component oxides is supported by the lower degree of reduction and
2
+
1+
different TPR patterns. Copper ions (Cu and/or Cu ) are probably incorporated into ceria
creating oxygen vacancies. This interface layer stabilizes easily reducible finely dispersed
CuO crystallites on the ceria surface, and their amount increases with the increase of Cu/Ce
ratio. We must also note that besides this copper species, higher copper content promotes the
segregation of bulk CuO nanoparticles on the outer silica surface, which do not interact with
CeO2.
1
9
Page 21 of 42

We calculated the theoretic catalytic conversion in ethyl acetate combustion, considering the
samples as a physical mixture of both oxides in appropriate ratio. At 600 K it was 65% and
5
9
4% for 2Cu4Ce and 4Cu2Ce, respectively, while the experimental values are about 31 and
0 %, respectively. The observed conversion for all supported bi-component materials is
about 1.5-2 times higher than the expected theoretic one. At the same time, the specific
activity, calculated per a unit catalyst surface is about twice lower for 2Cu4Ce as compared to
4
Cu2Ce, indicating that the observed effects are not only directed by the surface area of the
catalysts. Taking into account the discussion of the physicochemical data, we assume that the
penetration of copper ions into the ceria lattice leads to the creation of less active catalytic
sites, while the formation of easily reducible finely dispersed CuO crystallites promotes ethyl
acetate oxidation via Mars van Krevelen mechanism, where the release of lattice oxygen is of
primary importance [1]. We also demonstrated better catalytic activity and stability for bi-
component materials in toluene oxidation. Hu [3] proposed a mechanism, where the
adsorption of oxygen and toluene on the catalyst surface is considered as a key step. In our
study the data from cyclohexanol catalytic test evidence presence of stable Lewis acidic sites
in bi-component materials, which could participate as strong and stabile adsorption sites
during the toluene oxidation.
The XRD pattern of the used in the catalytic test samples evidences re-dispersion of metal
oxide phase and formation of reduced copper species, like Cu O and metallic copper (Fig. 4),
2
as was also reported in [82]. XPS analyses clearly demonstrate an increase of the relative part
4
+
of the reduced copper species with a simultaneous increase in the Ce amount after the
catalytic test. A decrease in the Cu/Ce surface ratio is established as well (Fig.7, Table 2). So,
the reduction transformations with copper ions in CuO nanoparticles could be assumed as
responsible for the observed catalysts deactivation. In accordance with TPR and cyclohexanol
analyses, such transformations concern in much lower extent the copper ions, penetrated into
2
0
Page 22 of 42

the Cu-Ce intermediate layer. However, we could not fully exclude the formation of non-
desorbable carbon containing products on the catalysts as alternative reason for their
deactivation, because the FTIR spectra of the samples after the catalytic test clearly
demonstrate accumulation of carbon containing products on the surface (Supplementary data,
Fig. S4).
Conclusion
Ceria doping with small amount of copper by co-precipitation of nitrate precursors leads to
significant incorporation of copper ions into the ceria lattice, which decreases their
reducibility and catalytic activity in VOCs oxidation. The increase in the Cu/Ce ratio for these
materials promotes the segregation of finely dispersed and easily reducible copper oxide
crystallites. These particles are stabilized, being in close contact with ceria and exhibits high
activity and selectivity in total oxidation of VOCs. Copper interaction with ceria significantly
changes after the metal oxides deposition on SBA-15 mesoporous silica. The silica support
controls the mode of interaction between the loaded metal oxide particles via regulation of
their dispersion and location into the porous structure. At low Cu/Ce ratio, the interaction
between different metal oxide species realizes predominantly into the mesopores of the silica
host matrix. The formation of finely dispersed copper and cerium oxide nanoparticles, which
are in close contact, ensures higher catalytic activity and selectivity of the supported bi-
component materials in comparison with the corresponding bulk analogues. The increase in
the Cu/Ce ratio leads to partial deposition of bulk copper oxide particles on the outer silica
surface, which restricts the contact with ceria. These particles contribute in a lower extent to
overall catalytic activity and as a result, lower catalytic activity and stability realizes for the
supported samples with higher Cu/Ce ratio as compared to the corresponding bulk ones. The
optimization of the samples composition and porous topology of the support seems to be a
2
1
Page 23 of 42

powerful factor for the preparation of highly active and selective copper and ceria bi-
component catalysts and further investigation is in progress.
Acknowledgements
Financial support of Bulgarian Academy of Science and National Scientific Fond of Ministry
of Education Projects DTK 02/64 and ДНТС/Китай 01/8, financial support from DGICYT in
Spain (Project CTQ-2009-14495) and bilateral project Bulgarian-Spain Inter-academic
Exchange Agreement (Project 2009BG0002) are acknowledged.
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Figure captions:
Figure 1. Evolution of ethyl acetate conversion (a, c) and CO selectivity (b, d) with reaction
2
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Figure 2. Ethyl acetate conversion (a, d) and selectivity to CO (b, f) with time on stream and
2
by-products distribution at 40 min. time on stream (c, e) at 630 K for bulk (left) and SBA-15
(right) modifications. Reaction conditions: 1.2 % ethyl acetate in air (170 ml/min).
Figure 3. Evolution of toluene conversion (a, c) and CO selectivity (b, d) with reaction
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temperature (left) or with time on stream at 630 K (right) for various SBA-15 modifications.
Reaction conditions: toluene partial pressure of 0.9 kPa in air (30 ml/min).
Figure 4. XRD patterns of SBA-15 modifications and bulk materials before (a,b) and after
catalytic test in ethyl acetate oxidation (c,d).
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